1. Field of the Invention
The invention is related to the field of pressure sensors and in particular to a multiple function pressure sensor generating signals indicative of the atmospheric pressure, a second variable pressure and a third pressure which is a function of the atmospheric pressure and the second pressure.
2. Prior Art
In operating systems such as electronic fuel control system for internal combustion engines, fuel requirement computations are often based of more than one pressure measurement. Very often these systems require inputs indicative of the pressure in the engine's air intakemanifold (MAP), the atmospheric pressure and the difference between the two. In a typical speed-density electronic fuel control system, the three measured pressures are used follows:
1. Manifold absolute pressure (MAP) is used for calculating the quantity of air being inhaled by the engine, which is determinative of the quantity of fuel to be injected into the engine under normal operating conditions. PA1 2. Atmospheric pressure is used to generate a correcton for altitude, adjusting the quantity of injected fuel as a function of atmospherc pressure, and PA1 3. Pressure differential between MAP and atmospheric pressure is used in the calculation of full load enrichment, under acceleration and high speed operation of the engine.
Normally these measurements are made by independent pressure sensors. Various types of pressure sensors are known in the art, such as electromagnetic pressure sensors in which the position of a magnetically permeable slug in a magnetic coil is varied as a function of pressure. The inductance of the coil as determined by the position of the slug, is indicative of the sensed pressure. Another type of pressure sensor utilizes a flexible diaphram acting as an electrode of a capacitor. Flexing of the diaphragm electrode as a function of sensed pressure changes the capacitance of the capacitor which may be used to produce a signal indicative of the sensed pressure. Another type of sensor is an aneroid sensor, in which the flexing of a diaphragm produces a mechanical motion indicative of the sensed pressure. Still another type of pressure sensor uses a strain gauge disposed along the surface of a flexible diaphragm which is strained as it is flexed by an applied pressure differential. In one form of this sensor the flexible diagphram is a silicon wafer having a strain gauge and integrated amplifier circuits diffused into the surface of the silicon wafer.
Basically, the above described sensors are single function devices capable of independently measuring any one of the desired pressure parameters. However, electronic fuel injection system of the existing types and those contemplated for the future require sensed pressure inputs of at least two of the parameters discussed above, and preferably the third measure input, also. To accommodate this requirement, it is necessary that at least two separate pressure sensors be incorporated into the control systems to provide the required input signals to the electronic fuel injection control unit.
The disclosed invention overcomes the requirement for multiple sensors, by combinating two independent pressure sensing elements sensing different parameters into one housing and electronically combining the outputs of the two sensing elements to generate the third required pressure signal. This substantially reduces number and cost of the sensors required as well as reducing the complexity and cost of the system. These cost savings become significant in high volume production items, such as those produced for the automotive market.